A suicide bomber killed 128 people at an election rally in southwestern Pakistan in the second election-related attack on Friday, officials said, amid growing tensions over ousted prime minister Nawaz Sharif's return ahead of the July 25 vote.

The bombing was the most deadly attack in Pakistan in over three years and is the third incident of election-related violence this week.

It came as Pakistan's caretaker government launched a crackdown on political gatherings. Sharif, who was ousted by the Supreme Court last year and convicted in absentia of corruption a week ago, arrived in the country to rally his party ahead of the general elections.

Baluchistan Home Minister Agha Omer Bangulzai told Reuters that the death toll in the attack had risen to 128 people, with over 150 wounded.

Senior police official Qaim Lashari had earlier said that more than 1,000 people were in attendance at the rally in the town of Mastung in the violence-plagued province of Baluchistan.

Islamist militants linked to the Taliban, al Qaeda and Islamic State have been operating in the province, which borders Iran as well as Afghanistan. It also has an indigenous ethnic Baluch insurgency fighting the central government.

Islamic State claimed responsibility for the attack, the group's AMAQ news agency said. The group provided no further detail or evidence for its claim.

In February 2017, Islamic State claimed responsibility for a suicide bombing at a shrine in southern Pakistan, killing 83 people and wounding over 150.

Among those killed in Friday's attack was Baluchistan provincial assembly candidate Siraj Raisani, whose brother Nawab Aslam Raisani had served as the provincial chief minister from 2008 to 2013.

His father, senior ANP leader Bashir Bilour, was killed in a suicide bombing in late 2012, in the run-up to Pakistan’s last election. 

- Reuters